I don't like Soho soundhouse. I went in there, and the sales assistant let me try out a couple of up-market ibanezes (J-custom) and a USA Jackson. Then I over-heard the manager (I was in the sound-proof booth, which apparently wasn't very soundproof) reprimanding the sales assistant, saying something along the lines of "You just don't let some random kid off the street come in and try some high price guitars!". Now (a) I'm 24 and (b) I had enough cash (at that time) to buy three of those guitars. The guitars were also REALLY poorly set up (I mean, I could do a better job, and I'm a hack). I don't like 'em, needless to say. Being in the centre of London, they're gonna have to be very sale-oriented, and obviously may have a problem with shoppers off the street coming in for a laugh, as it were, but that was ridiculous. Notwithstanding the fact that I'd come the whole way from Northern Ireland to try out their stuff...
Anyway, I strongly recommend the Wembley Guitar Centre. I went in, again, random kid off the street, and they let me try what I wanted. Were really nice and pleasant too, and never hurried me. They have a soundproof booth in the centre, so you can crank all you want (attenuating earplugs might be useful, lol), and I think they stock Engl, Marshall, Fender, Mesa, Ashdown, Hughes and Kettner, Laney, Peavey and Mesa (probably more).
The Oxford Guitar Gallery and Holiday Music in Leytonstone, East London, were also similarly great in letting me try out what I wanted, and were equally nice, but neither had a soundproof booth (they had separate testing rooms, but they weren't soundproof). So if you can only make one place, I'd say the Wembley Guitar Centre (from my experience).
:drink: